FinFET gate structure and method for fabricating the same

ABSTRACT

A semiconductor device includes a n-type gate structure over a first semiconductor fin, in which the n-type gate structure includes a n-type work function metal layer overlying the first high-k dielectric layer. The n-type work function metal layer includes a TiAl (titanium aluminum) alloy, in which an atom ratio of Ti (titanium) to Al (aluminum) is in a range substantially from 1 to 3. The semiconductor device further includes a p-type gate structure over a second semiconductor fin, in which the p-type gate structure includes a p-type work function metal layer overlying the second high-k dielectric layer. The p-type work function metal layer includes titanium nitride (TiN), in which an atom ratio of Ti to N (nitrogen) is in a range substantially from 1:0.9 to 1:1.1.

RELATED APPLICATIONS

This application is a continuation of U.S. Non-Provisional patent application Ser. No. 14/797,234, now issued as U.S. Pat. No. 9,620,610, titled “METHOD FOR FORMING SEMICONDUCTOR STRUCTURE”, and filed on Jul. 13, 2015, which is a divisional of U.S. Non-Provisional patent application Ser. No. 13/659,109, now issued as U.S. Pat. No. 9,082,770, titled “DAMASCENE GAP STRUCTURE”, and filed on Oct. 24, 2012. U.S. Non-Provisional patent application Ser. Nos. 14/797,234 and 13/659,109 are incorporated herein by reference.

BACKGROUND

The semiconductor integrated circuit (IC) industry has experienced rapid growth. In the course of the IC evolution, functional density (defined as the number of interconnected devices per chip area) has generally increased while geometry size (i.e., the smallest component (or line) that can be created using a fabrication process) has decreased. A scaling down process generally provides benefits by increasing production efficiency and lowering associated costs. But, such scaling down has increased the complexity of processing and manufacturing ICs. For these advances to be realized, similar developments in IC manufacturing are needed.

As the semiconductor IC industry has progressed into nanometer technology process nodes in pursuit of higher device density, higher performance, and lower costs, challenges from both fabrication and design have resulted in the development of three-dimensional (3D) devices such fin-like field effect transistors (FinFETs). Advantages of FinFET devices include reducing the short channel effect and higher current flow. There has been a desire to use a FinFET device with a high-k gate dielectric and metal gate electrode to improve device performance as feature sizes continue to decrease. A n-type MOS (NMOS) device and a p-type MOS (PMOS) device require different work functions for their respective gate structures. Conventional FinFET devices with high-k metal gates and methods of fabricating the FinFET devices have not been entirely satisfactory in all respects, especially for fabricating a NMOS device together with a PMOS device.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is noted that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.

FIG. 1 is a schematic cross-sectional view of a semiconductor device in accordance with some embodiments of the present disclosure.

FIG. 2A and FIG. 2B are schematic cross-sectional views of a semiconductor device in accordance with certain embodiments of the present disclosure.

FIG. 3A to FIG. 3G are schematic cross-sectional views of intermediate stages showing a method for fabricating a semiconductor device in accordance with some embodiments of the present disclosure.

FIG. 4 is a flow chart showing a method for fabricating a semiconductor device in accordance with some embodiments of the present disclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, or examples, for implementing different features of the provided subject matter. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact.

Terms used herein are only used to describe the specific embodiments, which are not used to limit the claims appended herewith. For example, unless limited otherwise, the term “one” or “the” of the single form may also represent the plural form. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly.

It will be understood that, although the terms “first”, “second”, “third”, etc., may be used in the claims to describe various elements, these elements should not be limited by these terms, and these elements correspondingly described in the embodiments are presented by different reference numbers. These terms are used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of the embodiments. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.

Embodiments of the present disclosure are directed to a semiconductor device on which a PMOS FinFET device and a NMOS FinFET device with metal gate structures are simultaneously formed, thereby simplifying the fabrication process. From an EDS (Energy dispersive spectroscopy) analysis, the NMOS FinFET device includes a n-type work function metal layer. The n-type work function metal layer includes a TiAl (titanium aluminum) alloy, in which an atom ratio of Ti (titanium) to Al (aluminum) is in a range substantially from 1 to 3, and both surfaces of the n-type work function metal layer contains an oxygen concentration substantially less than 10 atom percent (at %). The PMOS FinFET device a p-type work function metal layer overlying the second high-k dielectric layer. The p-type work function metal layer includes titanium nitride (TiN), in which an atom ratio of Ti to N (nitrogen) is in a range substantially from 1:0.9 to 1:1.1, and the p-type work function metal layer contains an oxygen concentration less than 10 atom percent (at %). Oxygen can cause a shift in the work function of the work function metal layer 146 b, and thus the less oxygen concentration can lead to the better quality of the work function metal layer. Therefore, embodiments of the present disclosure provide work function metal layers with excellent properties.

Referring to FIG. 1, FIG. 1 is a schematic cross-sectional diagram of a semiconductor device in accordance with some embodiments of the present disclosure. The semiconductor device includes a semiconductor substrate 102, a first semiconductor fin 110 a, a second semiconductor fin 110 b, a n-type gate structure 100 a and a p-type gate structure 100 b. The semiconductor fin 110 a and the semiconductor fin 110 b are disposed on the semiconductor substrate 102, and separated by an isolation structure 104. In some embodiments, the isolation structure 104 is a shallow trench isolation (STI). The semiconductor substrate 102 is defined as any construction including semiconductor materials, including, but is not limited to, bulk silicon, a semiconductor wafer, or a silicon germanium substrate. Other semiconductor materials including group III, group IV, and group V elements may also be used. The semiconductor fins 110 a and 110 b protrude from the semiconductor substrate 102. A gate spacer 122 a is formed on sidewalls of the n-type gate structure 100 a, and a gate spacer 122 b is formed on sidewalls of the p-type gate structure 100 b. The gate spacer 122 a, and the gate spacer 122 b may include silicon oxide, silicon nitride, silicon oxynitride, or other dielectric material. Source/drain portions 112 a and 114 a are disposed on the semiconductor fin 110 a adjacent to both sides of the gate spacer 122 a, and thus the source/drain portions 112 a and 114 a together with the n-type gate structure 100 a forms a NMOS FinFET device. Source/drain portions 112 b and 114 b are disposed on the semiconductor fin 110 b adjacent to both sides of the gate spacer 122 b, and thus the source/drain portions 112 b and 114 b together with the p-type gate structure 100 b forms a PMOS FinFET device. In some examples, the source/drain portions 112 a and 114 a include SiP, and the source/drain portions 112 b and 114 b include SiGe.

In some embodiments, an etching stop layer 120 overlies the gate spacer 122 a, the source/drain portions 112 a and 114 a, the isolation structure 104, the gate spacer 122 b, and the source/drain portions 112 b and 114 b. An inter-layer dielectric (ILD) 170 overlies the etching stop layer 120. The ILD 170 may include silicon oxide, phosphosilicate glass (PSG), borophosphosilicate glass (BPSG), and the like.

The n-type gate structure 100 a includes an initial layer 130 a, a high-k dielectric layer 140 a, a capping metal layer 142 a, a barrier metal layer 144 a, a TiN layer 146 a, a n-type work function metal layer 148 a, a blocking metal layer 150 a, and a metal filler 160 a. The initial layer 130 a is disposed over the semiconductor fin 110 a. In some examples, the initial layer 130 a includes a silicon oxide layer. The high-k dielectric layer 140 a is disposed over the initial layer 130 a, and is enclosed by the gate spacer 122 a. The high-k dielectric layer 140 a may have a thickness ranging from about 10 angstroms to about 20 angstroms. In some embodiments, the high-k dielectric layer 140 a may include a high-k dielectric such as hafnium oxide (HfO₂) hafnium silicon oxide (HfSiO), hafnium silicon oxynitride (HfSiON), hafnium tantalum oxide (HfTaO), hafnium titanium oxide (HfTiO), hafnium zirconium oxide (HfZrO), or combinations thereof.

The capping metal layer 142 a overlies the high-k dielectric layer 140 a, and is disposed between the high-k dielectric layer 140 a and the n-type work function metal layer 148 a. The capping metal layer 142 a includes TiN, and may have a thickness ranging from about 10 angstroms to about 30 angstroms. The barrier metal layer 144 a overlies the capping metal layer 142 a, and is disposed between the capping metal layer 142 a and the n-type work function metal layer 148 a. The barrier metal layer 144 a includes TaN (tantalum nitride) and may have a thickness ranging from about 10 angstroms to about 30 angstroms. The TiN layer 146 a overlies the barrier metal layer 144 a, and is disposed between the barrier metal layer 144 a and the n-type work function metal layer 148 a, and may have a thickness ranging from about 5 angstroms to about 20 angstroms. The capping metal layer 142 a, the barrier metal layer 144 a, and the TiN layer 146 a are used to prevent impurities from entering underlying layers. In certain embodiments, only one or more of the capping metal layer 142 a, the barrier metal layer 144 a, and the TiN layer 146 a is disposed between the high-k dielectric layer 140 a and the n-type work function metal layer 148 a. It is noted that the sequence of the capping metal layer 142 a, the barrier metal layer 144 a, and the TiN layer 146 a may be changed without affecting their purposes.

The n-type work function metal layer 148 a overlies the TiN layer 146 a and the high-k dielectric layer 140 a, and may have a thickness ranging from about 30 angstroms to about 100 angstroms. The n-type work function metal layer 148 a includes a TiAl (titanium aluminum) alloy, in which both surfaces of the n-type work function metal layer 148 a adjoin the TiN layer 146 a and the blocking metal layer 150 a respectively. From a result of EDS line scan, an atom ratio of Ti (titanium) to Al (aluminum) is in a range substantially from 1 to 3, and the both surfaces of the n-type work function metal layer 148 a contains an oxygen concentration less than about 10 atom percent (at %), and Al atom concentrations near or at the both surfaces of the n-type work function metal layer 148 a are higher than Al atom concentrations at other portions of the n-type work function metal layer 148 a, i.e. more Al segregation near or at the both surfaces of the n-type work function metal layer 148 a, thereby providing the work function metal layer with excellent properties. Oxygen can cause a shift in the work function of the n-type work function metal layer 148 a, and thus the less oxygen concentration can lead to the better quality of the n-type work function metal layer 148 a.

The blocking metal layer 150 a overlies the n-type work function metal layer 148 a for protecting the n-type work function metal layer 148 a, in which the blocking metal layer 150 a includes TiN, and may have a thickness ranging from about 10 angstroms to about 30 angstroms. The metal filler 160 a fills a trench (not labeled) peripherally enclosed by the blocking metal layer 150 a, and may have a thickness ranging from about 1000 angstroms to about 5000 angstroms. The metal filler 160 a can be configured to provide an electrical transmission. In some embodiments, the metal filler 160 a may be formed from materials such as tungsten, copper or other suitable materials, and/or combinations thereof.

The p-type gate structure 100 b includes an initial layer 130 b, a high-k dielectric layer 140 b, a capping metal layer 142 b, a barrier metal layer 144 b, a p-type work function metal layer 146 b, a TiAl layer 148 b a blocking metal layer 150 b, and a metal filler 160 b. The initial layer 130 b is disposed over the semiconductor fin 110 b. In some examples, the initial layer 130 b includes a silicon oxide layer. The high-k dielectric layer 140 b is disposed over the initial layer 130 b, and is enclosed by the gate spacer 122 b. The high-k dielectric layer 140 b may have a thickness ranging from about 10 angstroms to about 20 angstroms. In some embodiments, the high-k dielectric layer 140 a may include a high-k dielectric such as hafnium oxide (HfO₂) hafnium silicon oxide (HfSiO), hafnium silicon oxynitride (HfSiON), hafnium tantalum oxide (HfTaO), hafnium titanium oxide (HfTiO), hafnium zirconium oxide (HfZrO), or combinations thereof.

The capping metal layer 142 b overlies the high-k dielectric layer 140 b, and is disposed between the high-k dielectric layer 140 b and the p-type work function metal layer 146 b. The capping metal layer 142 b includes TiN, and may have a thickness ranging from about 10 angstroms to about 30 angstroms. The barrier metal layer 144 b overlies the capping metal layer 142 b, and is disposed between the capping metal layer 142 b and the p-type work function metal layer 146 b. The barrier metal layer 144 b includes TaN (tantalum nitride) and may have a thickness ranging from about 10 angstroms to about 30 angstroms. The capping metal layer 142 b and the barrier metal layer 144 b are used to prevent impurities from entering underlying layers. In certain embodiments, only one or more of the capping metal layer 142 b and the barrier metal layer 144 b is disposed between the high-k dielectric layer 140 b. It is noted that the sequence of the capping metal layer 142 b and the barrier metal layer 144 b may be changed without affecting their purposes.

The p-type work function metal layer 146 b overlies the barrier metal layer 144 b, and may have a thickness ranging from about 5 angstroms to about 20 angstroms. The p-type work function metal layer 146 b includes TiN, in which from a result of EDS line scan, an atom ratio of Ti to N (nitrogen) is in a range substantially from 1:0.9 to 1:1.1, and the p-type work function metal layer 146 b contains an oxygen concentration less than about 10 atom percent (at %), thereby providing the work function metal layer with excellent properties. Oxygen can cause a shift in the work function of the p-type work function metal layer 146 b, and thus the less oxygen concentration can lead to the better quality of the p-type work function metal layer 146 b.

The TiAl layer 148 b overlies the p-type work function metal layer 146 b, and is disposed between the p-type work function metal layer 146 b and the blocking metal layer 150 b, and may have a thickness ranging from about 30 angstroms to about 100 angstroms. The blocking metal layer 150 b overlies the TiAl layer 148 b for protecting the TiAl layer 148 b and the p-type work function metal layer 146 b, in which the blocking metal layer 150 b includes TiN, and may have a thickness ranging from about 10 angstroms to about 30 angstroms. The metal filler 160 b fills a trench (not labeled) peripherally enclosed by the blocking metal layer 150 b, and may have a thickness ranging from about 1000 angstroms to about 5000 angstroms. The metal filler 160 b can be configured to provide an electrical transmission. In some embodiments, the metal filler 160 b may be formed from materials such as tungsten, copper or other suitable materials, and/or combinations thereof.

The aforementioned high-k dielectric layers 140 a and 140 b may be formed from one identical layer; the aforementioned capping metal layers 142 a and 142 b may be formed from one identical layer; the aforementioned barrier metal layers 144 a and 144 b may be formed from one identical layer; the aforementioned TiN layer 146 a and p-type work function metal layer 146 b may be formed from one identical layer; the aforementioned n-type work function metal layer 148 b and TiAl layer 148 b may be formed from one identical layer; the aforementioned blocking metal layers 150 a and 150 b may be formed from one identical layer; and the aforementioned metal fillers 160 a and 160 b may be formed from one identical layer.

Referring to FIG. 2A and FIG. 2B, FIG. 2A and FIG. 2B are schematic cross-sectional views of a semiconductor device in accordance with certain embodiments of the present disclosure. The semiconductor device includes a semiconductor substrate 202, a semiconductor fin 210 a, a second semiconductor fin 210 b, a n-type gate structure 200 a and a p-type gate structure 200 b. The semiconductor fin 210 a and the semiconductor fin 210 b are disposed on the semiconductor substrate 202, and separated by an isolation structure 204. In some embodiments, the isolation structure 204 is a shallow trench isolation (STI). The semiconductor substrate 202 is defined as any construction including semiconductor materials, including, but is not limited to, bulk silicon, a semiconductor wafer, or a silicon germanium substrate. Other semiconductor materials including group III, group IV, and group V elements may also be used. The semiconductor fins 210 a and 210 b protrude from the semiconductor substrate 202. A gate spacer 222 a is formed on sidewalls of the n-type gate structure 100 a, and a gate spacer 222 b is formed on sidewalls of the p-type gate structure 200 b. The gate spacer 222 a, and the gate spacer 222 b may include silicon oxide, silicon nitride, silicon oxynitride, or other dielectric material. Source/drain portions 212 a and 214 a are disposed on the semiconductor fin 210 a adjacent to both sides of the gate spacer 222 a, and thus the source/drain portions 212 a and 214 a together with the n-type gate structure 200 a forms a NMOS FinFET device. Source/drain portions 212 b and 214 b are disposed on the semiconductor fin 210 b adjacent to both sides of the gate spacer 222 b, and thus the source/drain portions 212 b and 214 b together with the p-type gate structure 200 b forms a PMOS FinFET device. In some examples, the source/drain portions 212 a and 214 a include SiP, and the source/drain portions 212 b and 214 b include SiGe.

In some embodiments, an etching stop layer 220 overlies the gate spacer 222 a, the source/drain portions 212 a and 214 a, the isolation structure 204, the gate spacer 222 b, and the source/drain portions 212 b and 214 b. An inter-layer dielectric (ILD) 270 overlies the etching stop layer 220. The ILD 270 may include silicon oxide, phosphosilicate glass (PSG), borophosphosilicate glass (BPSG), and the like.

The n-type gate structure 200 a includes an initial layer 230 a enclosed by a gate spacer 222 a, and the p-type gate structure 200 b includes an initial layer 230 b enclosed by a gate spacer 222 b. Each of the n-type gate structure 200 a and the p-type gate structure 200 b includes a high-k dielectric layer 240, a TiN layer 242, a TaN 244, a TiN layer 246, a TiAl layer 248, a TiN layer 250, and a metal filler 260. The initial layer 230 a is disposed over the semiconductor fin 210 a, and the initial layer 230 b is disposed over the semiconductor fin 210 b. In some examples, each of the initial layer 230 a and the initial layer 230 b includes a silicon oxide layer. The high-k dielectric layer 240 is disposed over the initial layers 230 a and 230 b. The high-k dielectric layer 240 may have a thickness ranging from about 10 angstroms to about 20 angstroms. In some embodiments, the high-k dielectric layer 140 a may include a high-k dielectric such as hafnium oxide (HfO₂) hafnium silicon oxide (HfSiO), hafnium silicon oxynitride (HfSiON), hafnium tantalum oxide (HfTaO), hafnium titanium oxide (HfTiO), hafnium zirconium oxide (HfZrO), or combinations thereof.

The TiN layer 242 overlies the high-k dielectric layer 240, and may have a thickness ranging from about 10 angstroms to about 30 angstroms. The TaN layer 244 overlies the TiN layer 242, and may have a thickness ranging from about 10 angstroms to about 30 angstroms. The TiN layer 246 overlies the TaN layer 244, and may have a thickness ranging from about 5 angstroms to about 20 angstroms. The TiN layer 242, and the TaN layer 244 are used to prevent impurities from entering underlying layers. In certain embodiments, only one or more of the TiN layer 242, and the TaN layer 244 is disposed over the high-k dielectric layer 240. It is noted that the sequence of the TiN layer 242, and the TaN layer 244 may be changed without affecting their purposes.

The TiAl layer 248 overlies the TiN layer 246 and the high-k dielectric layer 240, and may have a thickness ranging from about 30 angstroms to about 100 angstroms. The blocking metal layer 250 overlies the TiAl layer 248 for protecting the underlying layers, and may have a thickness ranging from about 10 angstroms to about 30 angstroms. The metal filler 260 fills a trench (not labeled) peripherally enclosed by the blocking metal layer 250, and may have a thickness ranging from about 1000 angstroms to about 5000 angstroms. The metal filler 260 can be configured to provide an electrical transmission. In some embodiments, the metal filler 260 may be formed from materials such as tungsten, copper or other suitable materials, and/or combinations thereof. A chemical mechanical polishing (CMP) operation is performed on the metal filler 260 shown in FIG. 2A until the gate spacers 222 a and 222 b are exposed, as shown in FIG. 2B. Therefore, a NMOS FinFET device (the source/drain portions 212 a and 214 a and the n-type gate structure 200 a) and a PMOS FinFET device (the source/drain portions 212 b and 214 b together with the p-type gate structure 200 b) can be simultaneously formed, thereby simplifying the fabrication process.

The TiAl layer 248 enclosed by the gate spacer 222 a is a n-type work function metal layer, in which both surfaces of the n-type work function metal layer adjoin the TiN layer 246 and the blocking metal layer 250 respectively. From a result of EDS scan line, an atom ratio of Ti (titanium) to Al (aluminum) is in a range substantially from 1 to 3, and the both surfaces of the n-type work function metal layer contains an oxygen concentration substantially less than 10 atom percent (at %), and Al atom concentrations near or at the both surfaces of the n-type work function metal layer are higher than Al atom concentrations at other portions of the n-type work function metal layer, i.e. more Al segregation near or at the both surfaces of the n-type work function metal layer. The TiN layer 246 enclosed by the gate spacer 222 b is a p-type work function metal layer, in which an atom ratio of Ti to N (nitrogen) is in a range substantially from 1:0.9 to 1:1.1, and the p-type work function metal layer contains an oxygen concentration substantially less than 10 atom percent (at %). According to the above EDS features, the work function metal layers with excellent properties can be provided.

Referring to FIG. 3A to FIG. 3G, FIG. 3A to FIG. 3G are schematic cross-sectional views of intermediate stages showing a method for fabricating a semiconductor device in accordance with some embodiments of the present disclosure.

As shown in FIG. 3A, a semiconductor substrate 302 is provided, and is patterned and etched using a photolithography technique to form a semiconductor fin 310 a and a semiconductor fin 310 b which are separated by an isolation structure 304. The semiconductor substrate 310 is defined as any construction including semiconductor materials, including, but is not limited to, bulk silicon, a semiconductor wafer, or a silicon germanium substrate. Other semiconductor materials including group III, group IV, and group V elements may also be used. In some embodiments, a layer of photoresist material (not shown) is deposited over the semiconductor substrate 310, and is irradiated (exposed) in accordance with a desired pattern and developed to remove a portion of the photoresist material. The remaining photoresist material protects the underlying material from subsequent processing operation, such as etching. It should be noted that other masks, such as an oxide or silicon nitride mask, may also be used in the etching process. In other embodiments, the semiconductor fin 310 a and the semiconductor fin 310 b may be epitaxially grown. For example, exposed portions of an underlying material, such as an exposed portion of the semiconductor substrate 210, may be used in an epitaxial process to form the semiconductor fin 310 a and the semiconductor fin 310 b. A mask may be used to control the shape of the semiconductor fin 310 a and the semiconductor fin 310 b during the epitaxial growth process.

A poly gate 380 a is formed on the semiconductor fin 310 a, and a poly gate 380 b is formed on the semiconductor fin 310 b. A gate spacer 322 a is formed on sidewalls of the poly gate 380 a, and a gate spacer 322 b is formed on sidewalls of the poly gate 380 b. The gate spacer 322 a, and the gate spacer 322 b may include silicon oxide, silicon nitride, silicon oxynitride, or other dielectric material. Source/drain portions 312 a and 314 a are formed on the semiconductor fin 310 a adjacent to both sides of the gate spacer 322 a. Source/drain portions 312 b and 314 b are formed on the semiconductor fin 310 b adjacent to both sides of the gate spacer 322 b. In some examples, the source/drain portions 312 a and 314 a include SiP, and the source/drain portions 312 b and 314 b include SiGe. In some embodiments, an etching stop layer 320 is formed over the gate spacer 322 a, the source/drain portions 312 a and 314 a, the isolation structure 304, the gate spacer 322 b, and the source/drain portions 312 b and 314 b. An inter-layer dielectric (ILD) 370 is formed over the etching stop layer 320. The ILD 370 may include silicon oxide, phosphosilicate glass (PSG), borophosphosilicate glass (BPSG), and the like.

Thereafter, as shown in FIG. 3B, a portion of the ILD 370 is removed to expose the etching stop layer 320 by, for example, wet or dry etching. Then, as shown in FIG. 3C, the etching stop layer 320 and the poly gates 380 a and 380 b are removed by, for example, wet or dry etching. Thereafter, as shown in FIG. 3D, an initial layer 330 a formed over the semiconductor fin 310 a, and the initial layer 330 b is formed over the semiconductor fin 310 b. In some examples, each of the initial layer 330 a and the initial layer 330 b includes a silicon oxide layer, may be formed by using chemical vapor deposition (CVD), thermal oxide, ozone oxidation, other suitable processes.

Then, as shown in FIG. 3E, a high-k dielectric layer 340 is formed over the initial layers 330 a and 330 b by using atomic layer deposition (ALD) or another suitable technique. The high-k dielectric layer 240 may have a thickness ranging from about 10 angstroms to about 20 angstroms. In some embodiments, the high-k dielectric layer 140 a may include a high-k dielectric such as hafnium oxide (HfO₂) hafnium silicon oxide (HfSiO), hafnium silicon oxynitride (HfSiON), hafnium tantalum oxide (HfTaO), hafnium titanium oxide (HfTiO), hafnium zirconium oxide (HfZrO), or combinations thereof.

A TiN layer 342 is formed over the high-k dielectric layer 340 by using ALD or another suitable technique, and may have a thickness ranging from about 10 angstroms to about 30 angstroms. A TaN layer 344 is formed over the TiN layer 342 by using ALD or another suitable technique, and may have a thickness ranging from about 10 angstroms to about 30 angstroms. A TiN layer 346 formed over the TaN layer 344 by using ALD or another suitable technique, and may have a thickness ranging from about 5 angstroms to about 20 angstroms. The TiN layer 342, and the TaN layer 344 are used to prevent impurities from entering underlying layers. In certain embodiments, only one or more of the TiN layer 342, and the TaN layer 344 is formed over the high-k dielectric layer 240. It is noted that the sequence of the TiN layer 342, and the TaN layer 344 may be changed without affecting their purposes. A TiAl layer 348 formed over the TiN layer 346 and the high-k dielectric layer 340 by using ALD or another suitable technique, and may have a thickness ranging from about 30 angstroms to about 100 angstroms. A blocking metal layer 350 formed over the TiAl layer 348 by using ALD or another suitable technique for protecting the underlying layers, and may have a thickness ranging from about 10 angstroms to about 30 angstroms.

Then, as shown in FIG. 3F, a metal filler 360 fills a trench (not labeled) peripherally enclosed by the blocking metal layer 350 by using CVD, ALD or another suitable technique. The metal filler 360 can be configured to provide an electrical transmission. In some embodiments, the metal filler 360 may be formed from materials such as tungsten, copper or other suitable materials, and/or combinations thereof.

Thereafter, as shown in FIG. 3G, a chemical mechanical polishing (CMP) operation is performed on the metal filler 360 until the gate spacers 322 a and 322 b are exposed. The metal filler 360 may have a thickness ranging from about 1000 angstroms to about 5000 angstroms Therefore, a NMOS FinFET device (the source/drain portions 312 a and 314 a and the n-type gate structure enclosed by the gate spacer 322 a) and a PMOS FinFET device (the source/drain portions 312 b and 314 b together and the p-type gate structure enclosed by the gate spacer 322 a) can be simultaneously formed, thereby simplifying the fabrication process.

The TiAl layer 348 enclosed by the gate spacer 322 a is a n-type work function metal layer, in which both surfaces of the n-type work function metal layer adjoin the TiN layer 346 and the blocking metal layer 350 respectively. From a result of EDS scan line, an atom ratio of Ti (titanium) to Al (aluminum) is in a range substantially from 1 to 3, and the both surfaces of the n-type work function metal layer contains an oxygen concentration substantially less than 10 atom percent (at %), and Al atom concentrations near or at the both surfaces of the TiAl layer 348 are higher than Al atom concentrations at other portions of the TiAl layer 348, i.e. more Al segregation near or at the both surfaces of the TiAl layer 348. The TiN layer 346 enclosed by the gate spacer 322 b is a p-type work function metal layer, in which an atom ratio of Ti to N (nitrogen) is in a range substantially from 1:0.9 to 1:1.1, and the p-type work function metal layer contains an oxygen concentration substantially less than 10 atom percent (at %). According to the above EDS features, the work function metal layers with excellent properties can be provided.

Referring to FIG. 4 and FIG. 3G, FIG. 4 is a flow chart showing a method for fabricating a semiconductor device in accordance with some embodiments of the present disclosure. The method begins at operation 400, in which a semiconductor fin 310 a (first semiconductor fin) and a semiconductor fin 310 b (second semiconductor fin) are formed on a semiconductor substrate 302, and are separated by an isolation structure 304. At operation 410, an initial layer 330 a (first initial layer) enclosed by a gate spacer 322 a (first gate spacer) is formed over the semiconductor fin 310 a, and an initial layer 330 b (second initial layer) enclosed by a gate spacer 322 b (second gate spacer) is formed over the semiconductor fin 310 b. At operation 420, a high-k dielectric layer 340 is formed over the initial layers 330 a and 330 b. At operation 430, a TiN layer 342 (first TiN layer) is formed over the high-k dielectric layer 320. At operation 440, a TaN layer 344 overlying the TiN layer 342. At operation 450, a TiN layer 346 (second TiN layer) is formed over the TaN layer 344. At operation 460, a TiAl layer 348 is formed over the TiN layer 346. At operation 470, a TiN layer 350 (third TiN layer) is formed over the TiAl layer 348. At operation 480, a metal filler 360 peripherally enclosed by the TiN layer 350 is formed. The TiAl layer 348 enclosed by the gate spacer 322 a acts as a n-type work function metal layer, in which an atom ratio of Ti (titanium) to Al (aluminum) is in a range substantially from 1 to 3, and both surfaces of the n-type work function metal layer contains an oxygen concentration substantially less than 10 atom percent (at %), and Al atom concentrations near or at the both surfaces of the TiAl layer 348 are higher than Al atom concentrations at other portions of the TiAl layer 348, i.e. more Al segregation near or at the both surfaces of the TiAl layer 348. The TiN layer 346 enclosed by the gate spacer 322 b acts as a p-type work function metal layer, wherein an atom ratio of Ti to N (nitrogen) is in a range substantially from 1:0.9 to 1:1.1, and the p-type work function metal layer contains an oxygen concentration substantially less than 10 atom percent (at %).

In accordance with some embodiments, a semiconductor device includes a semiconductor substrate; a first semiconductor fin on the semiconductor substrate; a n-type gate structure over the first semiconductor fin; a blocking metal layer overlying the n-type work function metal layer; and a first metal filler peripherally enclosed by the blocking metal layer. The blocking metal layer includes TiN (titanium nitride). The n-type gate structure includes a first initial layer over the first semiconductor fin; a first high-k dielectric layer over the first initial layer and enclosed by a first gate spacer; and a n-type work function metal layer overlying the first high-k dielectric layer, the n-type work function metal layer comprising a TiAl (titanium aluminum) alloy, in which an atom ratio of Ti (titanium) to Al (aluminum) is in a range substantially from 1 to 3.

In accordance with certain embodiments, a semiconductor device includes a semiconductor substrate; a first semiconductor fin and a second semiconductor fin on the semiconductor substrate; a n-type gate structure; and a p-type gate structure. The first semiconductor fin and the second semiconductor fin are separated by an isolation structure. The n-type gate structure includes a first initial layer over the first semiconductor fin and enclosed by a first gate spacer, and the p-type gate structure includes a second initial layer over the second semiconductor fin and enclosed by a second gate spacer. Each of the n-type gate structure and the p-type gate structure includes a high-k dielectric layer over the first initial layer and the second initial layer; a first TiN layer overlying the high-k dielectric layer; a TaN layer overlying the first TiN layer; a second TiN layer overlying the TaN layer; a TiAl layer overlying the second TiN layer; and a third TiN layer overlying the TiAl layer; and a metal filler peripherally enclosed by the third TiN layer. The TiAl layer enclosed by the first gate spacer is a n-type work function metal layer, in which an atom ratio of Ti (titanium) to Al (aluminum) is in a range substantially from 1 to 3. The second TiN layer enclosed by the second gate spacer is a p-type work function metal layer, in which an atom ratio of Ti to N (nitrogen) is in a range substantially from 1:0.9 to 1:1.1.

In accordance with some embodiments, a method includes forming a first semiconductor fin and a second semiconductor fin on a semiconductor substrate, in which the first semiconductor fin and the second semiconductor fin are separated by an isolation structure. A first initial layer enclosed by a first gate spacer over the first semiconductor fin is formed, and a second initial layer enclosed by a second gate spacer over the second semiconductor fin is formed. A high-k dielectric layer is formed over the first initial layer and the second initial layer. A first TiN layer is formed over the high-k dielectric layer. A TaN layer is formed over the first TiN layer. A second TiN layer is formed over the TaN layer. A TiAl layer is formed over the second TiN layer. A third TiN layer is formed over the TiAl layer. A metal filler peripherally enclosed by the third TiN layer is formed. The TiAl layer enclosed by the first gate spacer acts as a n-type work function metal layer, in which an atom ratio of Ti (titanium) to Al (aluminum) is in a range substantially from 1 to 3. The second TiN layer enclosed by the second gate spacer acts as a p-type work function metal layer, in which an atom ratio of Ti to N (nitrogen) is in a range substantially from 1:0.9 to 1:1.1.

The foregoing outlines features of several embodiments so that those skilled in the art may better understand the aspects of the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure. 

What is claimed is:
 1. A semiconductor device, comprising: a semiconductor substrate; a first semiconductor fin on the semiconductor substrate; and a n-type gate structure over the first semiconductor fin, wherein the n-type gate structure comprises: a first initial layer over the first semiconductor fin; a first high-k dielectric layer over the first initial layer and enclosed by a first gate spacer; a n-type work function metal layer over the first high-k dielectric layer, wherein an atomic concentration of a metal within a first region of the n-type work function metal layer is different than an atomic concentration of the metal within a second region of the n-type work function metal layer; a first blocking metal layer over the n-type work function metal layer; and a first metal filler peripherally enclosed by the first blocking metal layer.
 2. The semiconductor device of claim 1, wherein the first high-k dielectric layer is in contact with a sidewall of the first gate spacer.
 3. The semiconductor device of claim 2, wherein the first initial layer is in contact with the sidewall of the first gate spacer.
 4. The semiconductor device of claim 1, wherein the n-type work function metal layer comprises TiAl.
 5. The semiconductor device of claim 1, wherein the metal is aluminum.
 6. The semiconductor device of claim 1, wherein the first region corresponds to a first surface region facing the first blocking metal layer.
 7. The semiconductor device of claim 1, wherein: an atomic concentration of the metal within a third region of the n-type work function metal layer is different than the atomic concentration of the metal within the second region of the n-type work function metal layer, and the second region is between the first region and the third region.
 8. The semiconductor device of claim 7, wherein: the first region corresponds to a first surface region facing the first blocking metal layer, and the third region corresponds to a second surface region facing the first high-k dielectric layer.
 9. The semiconductor device of claim 8, wherein: the n-type work function metal layer comprises a TiAl, and the metal is aluminum.
 10. The semiconductor device of claim 1, comprising: a p-type gate structure over a second semiconductor fin, wherein the p-type gate structure comprises a p-type work function metal layer over the second semiconductor fin.
 11. The semiconductor device of claim 10, comprising: a TiN layer between the first high-k dielectric layer and the n-type work function metal layer, wherein the TiN layer has a same composition as the p-type work function metal layer.
 12. The semiconductor device of claim 10, wherein the p-type gate structure comprises a TiAl layer, wherein the n-type work function metal layer has a same composition as the TiAl layer.
 13. The semiconductor device of claim 1, wherein the n-type gate structure comprises a capping metal layer disposed between the first high-k dielectric layer and the n-type work function metal layer.
 14. The semiconductor device of claim 13, wherein the n-type gate structure comprises a barrier metal layer disposed between the capping metal layer and the n-type work function metal layer.
 15. The semiconductor device of claim 14, wherein the n-type gate structure comprises a TiN layer disposed between the barrier metal layer and the n-type work function metal layer.
 16. The semiconductor device of claim 15, wherein the capping metal layer comprises TiN and the barrier metal layer comprises tantalum nitride.
 17. A method for forming a semiconductor device, the method comprising: forming a first semiconductor fin on a semiconductor substrate; forming a first initial layer over the first semiconductor fin; forming a first high-k dielectric layer over the first initial layer, wherein the first high-k dielectric layer is enclosed by a first gate spacer; forming a n-type work function metal layer over the first high-k dielectric layer, wherein an atomic concentration of a metal within a first region of the n-type work function metal layer is different than an atomic concentration of the metal within a second region of the n-type work function metal layer; and forming a first blocking metal layer over the n-type work function metal layer, wherein a first metal filler is peripherally enclosed by the first blocking metal layer.
 18. The method of claim 17, comprising: forming a TiN layer over the first high-k dielectric layer before forming the n-type work function metal layer.
 19. The method of claim 17, comprising: forming an etch stop layer over the first gate spacer before forming the first initial layer.
 20. A semiconductor device, comprising: a semiconductor substrate; a first semiconductor fin on the semiconductor substrate and a second semiconductor fin on the semiconductor substrate; and a n-type gate structure over the first semiconductor fin and a p-type gate structure over the second semiconductor fin, wherein each of the n-type gate structure and the p-type gate structure comprise: a first initial layer over the first semiconductor fin; a first high-k dielectric layer over the first initial layer and enclosed by a first gate spacer; a TiN layer over the first high-k dielectric layer; a TiAl layer over the TiN layer; a first blocking metal layer over the TiAl layer; and a first metal filler peripherally enclosed by the first blocking metal layer. 